Image suppression in superheterodyne receivers



343331131943. F. H. SCHEER 2,313,032

IMAGE SUPPRESSION IN SUPERHETERODYNE RECEIVERS Filed Oct. 1, 1941 2Sheets-Sheet 1 75 05c/Lmr02 fTu/vee INVENTOR FEL'DEE/Ck 52 ATTORNEY Mart2h 2, 1943. F SCHEER 2,313,032

IMAGE SUPPRESSION IN SUPERHETERODYNE RECEIVERS Filed Oct. 1, 1941 2Sheet s-Sheet .2

Patented Mar. 2, 1943 WAGE SUPPRESSION IN SUPER- HETERODYNE RECEIVERSFrederick H. Scheer, Kenmore, N. Y., assignor to Colonial RadioCorporation, Buffalo, N. Y., a corporation of New York ApplicationOctober 1, 1941, Serial No. 413,170

13 Claims.

'This invention relates to radio receiving apparatus and moreparticularly to signal selecting circuits for superheterodyne receivers.

In such receivers, as is well known, the inter mediate frequencyamplifier stage or stages are tuned to a frequency above audibility andusually below the lowest frequency of the broadcast band. Presentpractice is to tune the intermediate frequency stages to 455 kc. inbroadcast receivers. Incoming radio frequency signals are heterodyned orbeat with an oscillation produced by the local oscillator. This givesrise to oscillations of a frequency which may be either the sum or thedifference between the incoming frequency and that of the localoscillator. Only those oscillations thereby produced Which have afrequency which enables them to pass through the intermediate frequencyamplifier are then amplified.

For example, if th intermediate frequency am.- plifier is tuned to 455kc. and a 540 kc. signal is being received and the local oscillator has,when this particular signal is being received, a frequency of 995 kc.,then there will be produced a beat frequency of 455 kc. after detectionby the action of the oscillation on the desired signal. However, ifthere is also an incoming signal of 1450 kc., at the same time, a newoscillation will be produced by the heterodyne action between the localoscillationand the 1450 kc. signal. This new oscillation Will, likewise,have a frequency of 455 kc. and will, accordingly, pass through theintermediate frequency amplifier, resulting in interference and crosstalk with the desired signal. The frequency of the undesired signal istermed the image frequency and it is equal to the frequency of thedesired signal plus twice the intermediate frequency.

It is usual to provide a signal selecting circuit in advance of thefirst detector in a superheter- .odyne and this circuit is ordinarilydesigned to be tuned to the frequency of the desired signal. However,when such a circuit is used, image interference may still be encounteredand it has been proposed in the past to eliminate or reduce this imageinterference by providing preselecting circuits and the like and also byproviding one or more wave traps associated with the antenna andvariably tuned.

The employment of wave traps for image suppression according to theprior art increases the cost of the receiver since it is necessary toprovide an additional coil and an additional con-denser, one or theother of which is usually variable. The provision of such additionalparts increases the cost of the receiver from the standpoint ofmaterials and parts employed and also the labor of assembling. J 1

In accordance with the co-pending application of Howard J. Benner,Serial No. 239,788 filed November 10, 1938, it is pointed out that it ispossible to provide a simple tuning circuit with only one variabletuning reactance which acts simultaneously as a selector of desiredfrequency and as an image suppressor; that is to say, the variation ofthe variable reactanoe causes the circuit to select the desired signaland at the same time to present an extremely high effective impedance tothe image signal. In the circuit of the aforesaid Benner application itis pointed out that there may occur a certain amount of mistrackingbetween the signal tuning and the image tuning. This mistracking is dueto the fact that the 9-1 variation of inductance in the tuned circuit,which is necessary to cover the 3-1 frequency range usually required ina broadcast receiver, also varies the image tuning in a 3-1 frequencyratio.

In the example above given with the signal selector tuned to 54.0 kc.the image frequency is 1450. With the receiver tuned to 1500 kc. theimage frequency is 2410 kc. However, in the Benner circuit abovereferred to, if the receiver is tuned to 1500 kc., the image suppressorwould be tuned to 4028 kc. instead of 2410, the true image frequency;that is to say, as the receiver is tuned from 540 to 1500 kc., thetuning of the image suppressor circuit increases too rapidly. In mostcases this is not a serious matter because the Worst image interferenceis usually found when the receiver is tuned to the low frequency end ofthe broadcast band and the image suppressor may be peaked so that it isproperly tracked when the receiver is tuned to the low frequency end ofthe broadcast band.

However, in case this departure from the theoretical value of the imagesuppressor is desired to be reduced or eliminated, I have discoveredthat it may be done very easily and effectively Without the use ofadditional coils or variable condensers.

Among the objects of my invention are:

To provide an image suppression circuit which does not require separatewave trap or preselector I circuits and in which the image suppressorcircuit may be made to track very exactly with its theoretical value.

To provide such a circuit which does not require the employment ofadditional coils or variable condensers as ordinarily employed.

Still another object of my invention is to provide such a circuit inwhich the image circuit may be made to track very closely with itstheoretical value by the employment of a conducting plate which may bemechanically connected with the tuning mechanism and moved into positionadjacent the input coil as the iron core customarily used-inpermeability tuning is moved out of the coil.

Still other objects and advantages of my invention will be apparent fromthe specification.

In this application I have particularly pointed out and distinctlyclaimed the part, improvement, or combination which I claim as myinvention or discovery, and I have explained the principles thereof andthe best mode in which I have contemplated applying those principles soas to distinguish my invention from other inventions. In the drawings:

Figure 1 is a circuit diagram of one form of circuit in accordance withmy invention.

Figure 2 is a set of curves, showing the variaticn of capacity necessaryto keep the image tuning exactly tracked.

Figure 3 is a plan view of the tuning mechanism including coil, core,capacity plate or sleeve, and driving cord; and

Figure 4.- is a developed view of the capacity plate employed.

Referring now more particularly to Figure 1, l designates the antenna ofa superheterodyne receiver which may be connected to ground throughantenna coupling condenser 2. The antenna coil 3 may be tuned by core 4of magnetic material which may be moved into and out of the coil 3. Suchcores may be satisfactorily formed of iron dust mixed with a suitableinsulating and binding material and pressed 01' otherwise processed toform what appears as a solid bar. Since such cores are easily obtainableon the market and their construction is per se not a part of myinvention, they are not described in detail.

The antenna I may be connected to one end of 'thecoil 3, the other endof which may be connected to the control electrode of detector oramplifier tube '1. AVG resistor II maybe employed in any suitableautomatic volume control circuit. Connected from the grid side of thecoil 3, I may provide condenser 6 to ground and this condenser may bethe usual high frequency trimmer ordinarily employed for aligning thereceiver. The cathode of tube 1 may be connected to ground throughsuitable resistance 12 bypassed by condenser [3.

If the core 4 is moved axially into and out of coil 3, there will beobserved a variation in the inductance of coil 3 and a variation of theresonant frequency of the circuit of the input circuit as a whole. Bymoving the core 4 in and out of the coil the receiver can be tuned tothe desired signal.

With most cores and coils commercially available, intended to cover thebroadcast range, the frequency of signal tuning is not a straight linefunction of core movement. This will be seen by reference to dot anddash curve A of Figure 2, showing signal frequency in kilocycles plottedagainst core travel in inches, and it will be observed that as the coreis withdrawn from fully inserted position, the frequency increases quiteslowly at first, then more rapidly, reaching a maximum rate of increase,and then decreases more slowly.

The image frequency of course is another curve parallel to curve A anddisplaced vertically above it by twice the intermediate frequency, andthe actual image frequency to which coil 3 and condenser are tuned maybe arranged by trimming or adjusting condenser 5 to coincide with thetrue image frequency at any point on the tuning range desired.Preferably, however, this is done at or near the low frequency end ofthe range; and if this is done, the resonant frequency of the imagesuppressor formed by coil 3 and condenser 5 increases at a faster ratethan desired, as already pointed out.

I have found that this increase can be prevented without the use ofadditional cores or condensers by providing a conducting plate, such asshown in Figure 3, which may be in the form of a sleeve, and which movesinto or over the coil as the core is moved out of the coil. This sleeve15 may be tied to the core mechanically and arranged to be moved by thesame mechanism which moves the core for tuning, such as cord 16, whichmay be attached to the sleeve 5 at the end remote from the coil by plug16a. It is possible for any given value of inductance 3 and capacity 5,which may be either a physical con denser in parallel with the coil 3 ormay be the distributed capacity of the coil 3, to calculate theadditional capacity required at any setting of the core to make theimage suppressor track exactly at its theoretical value.

Assuming it is desired to cover a signal ran e of 540 to 16 20 kc., itis seen immediately that this frequency range squared=is equal to 9 and,therefore, the inductance at 540 kc. must be 9 times that at 1620 kc.The image frequency at 1620 kc. is 2530 kc.. and that at 540 is 1450,assuming an intermediate frequency of 455 kc. The ratio of imagefrequency change squared is equal to 3.05. For a signal frequency of 540the constants of the circuit made up by coil 3 and capacity 5 are givenby the equation where L1 is the inductance of coil 3 with the core fullyinserted and C1 the value of capacity 5.

At any higher frequency the constants are given by equation where ACrepresents the additional capacity introduced in parallel with capacity5 by the sleeve l5. Solving for A0 in terms of 01 it is found thatc=1.96 C1 for a signal frequency of 1620. AC may be calculated for anyfrequency between the lowest and highest. The following table shows thecalculation of values for a series of frequencies.

C1=original capacity necessary to tune antiresonant circuit to imagefrequency.

L1 f1 2 (1460) 2 AG g Signal F; L, Ima efz f3 (2) (4) C Plotting AC/Cfor the various frequencies calculated, as shown by curve A, gives curveB (full line). Comparing curve B with curve C' (dotted) I that thecapacity increase required to track the image circuit exactly isconsiderably less than linear at the beginning; that is, the capacityshould increase rather slowly at first. Thereafter it begins to increaseat a faster rate and continues to increase at a faster rate until about1650 kc. and thereafter should increase more slowly. This manner ofcapacity change may be simply provided by cutting the plate I as shownin Figure 4 and thereafter rolling it into the form of a cylinder asindicated in Figure 3 and arranging it to move into the coil asindicated in Figure 3 so that the spread end of the sleeve enters thecoil first.

The exact diameter of the sleeve and the clearance between the sleeveand the coil and the angle of taper of the sleeve before rolling maybest be determined by trial, using a series of cardboard cylinders ofdifferent diameters coated with foil, the foil being cut away by trialand the image ratio being measured to determine the maximum image ratioat a series of chosen frequencies. ihe maximum image ratio will occurwhen the image suppressor circuit, consisting of coil 3 and capacity 5,is exactly tuned to the theoretical image value. After the optimumdiameter and taper has been determined, the foil may be unrolled andused as a pattern or template for the sleeve l5 in case quantityproduct-ion is desired. The sleeve may be made of any suitable material,such as copper or aluminum.

For all ordinary values of inductance 3 and capacity 5 encountered inthe design of broadcast receivers the effect of the sleeve 55 on thetuning of the signal circuit is for all practical purposes negligible,although, if the circuit is trimmed by the high frequency trimmer 6before the addition of the sleeve 15, it may be necessary to slightlyreadjust trimmer 6 after sleeve l5 has been added.

In some cases, where it is desired to have the sleeve l5 introduce alarger capacity than is obtainable with minimum clearance between thesleeve and the coil 3, the sleeve !5 may be connected electrically tothe grid end of coil 3.

While I have shown and described certain preferred embodiments of myinvention, it will be understood that modifications and changes may bemade without departing from the spirit and scope of my invention.

I claim:

1. An image suppression system for superheterodyne radio receivershaving an antenna and a vacuum tube with a control electrode, includinga first resonant circuit, a portion of which is connected between saidcontrol electrode and ground; and a second resonant circuit in seriesbetween said antenna and said control electrode, said resonant circuitshaving a common variable inductance and being tuned respectively to asignal frequency and to the corresponding image frequency; and aconducting plate movable with respect to said inductance to vary thedistributed capacity thereof, said plate being arranged to increase thedistributed capacity of said inductance as the inductance is reduced.

2. An image suppression system for superheterodyne radio receivershaving an antenna and a vacuum tube With a control electrode, includinga first resonant circuit, a portion of which is connected between saidcontrol electrode and ground; and a second resonant circuit in seriesbetween said antenna and said control electrode, said resonant circuitshaving a common variable inductance device comprising an inductance coiland a ferro-magnetic core movable relatively thereto and being tunablesimultaneously over substantially different frequency ranges by motionof said core; and a conducting plate movable with said core relativelyto said coil to change the distributed capacity thereof, said platebeing arranged to increase the distributed capacity of said coil as theinductance is reduced.

3. An image suppression system for superheterodyne radio receivershaving an antenna and a vacuum tube with a control electrode including afirst resonant circuit, a portion of which is connected between saidcontrol electrode and ground; and a second resonant circuit in seriesbetween said antenna and said control electrode, said resonant circuitshaving a common variable inductance device comprising an inductance coiland a ferro-magnetic core movable relatively thereto; and a conductingplate movable with said core relatively to said coil to vary thedistributed capacity thereof and being movable in a direction toincrease the distributed capacity thereof with reduction of theinductance thereof, said resonant circuits being simultaneously tuned bymotion of said core and said plate to a signal frequency and to thecorresponding image frequency.

4. A preselector system for superheterodyne radio receivers including avariable inductance comprising an inductance coil, a movableferromagnetic core, and a movable conducting plate, said core and saidplate being so arranged that said plate, approaches said coil as saidcore is withdrawn therefrom, first and second capacitors connected withsaid inductance device to secure resonance within a first range offrequencies by movement of said core; and a third capacitor in shuntwith said inductance device to secure resonance within a second range offrequencies by movement of said core, said capacitors being so chosenthat any frequency of said second range is higher than the correspondingfrequency of the first range by a practically constant amount throughoutsaid core movement.

5. A preselector system for superheterodyne radio receivers including avariable inductance device comprising an inductance coil, a movableferro-magnetic core, and a movable conducting plate arranged to movecloser to said coil as said core is moved out of said coil; first andsecond capacitors in series shunted across said inductance device tosecure resonance within a first range of frequencies by movement of saidcore and said conducting plate; and a third capacitor in shunt with saidinductance device to secure resonance within a second range offrequencies by movement of said core and said conducting plate,-saidcapacitors being so chosen that any frequency of said second range ishigher than the corresponding frequency of said first range by apractically constant amount throughout said core and plate movement.

6. In a superheterodyne radio receiving system an antenna; a vacuum tubehaving an input electrode; and elements including a variable inductancedevice comprising an inductance coil, a movable ferro-magnetic core, anda movable conducting plate arranged to approach said coil as the core iswithdrawn therefrom; a first resonant circuit including said inductancedevice and tunabie thereby over a first range of frequencies connectedbetween said antenna and ground; and a second resonant circuit includingsaid inductance device and tunable thereby over a second frequency rangein series between said antenna and said input electrode, said elementsbeing so proportioned that said resonant circuits are maintained at apractically constant frequency difierence during movement of said core.

An image suppression system for superheterodyne radio receivers havingan antenna and a vacuum tube with a control electrode, including a firstresonant circuit, a portion of which is connected between said controlelectrode and ground; and a second resonant circuit in series betweensaid antenna and said control electrode, said resonant circuits having acommon variable inductance device comprising an inductance coil, aferro-magnetic core movable relatively thereto, and a conducting membermechanically movable with said core and movable toward said coil as saidcore moves out, said member being tapered to provide an increased rateof capacity change of said coil as it moves toward said coil, saidresonant circuits being simultaneously tuned by motion of said core andsaid plate to a signal frequency and the corresponding image frequency.

8. An image suppression system for superheterody-ne radio receivershaving an antenna and a vacuum tube with a control electrode, includinga first resonant circuit, a portion of which is connected between saidcontrol electrode and ground; and a second resonant circuit in seriesbetween said antenna and said control electrode, said resonant circuitshaving a common variable inductance device comprising an inductancecoil, a ferro-magnetic core movable relatively thereto; and a conductingsleeve movable with said core relatively to said coil to vary thedistributed capacity thereof, and being movable in a direction toincrease the distributed capacity thereof with reduction of theinductance thereof, said resonant circuits bein simultaneously tuned bymotion of said core'and said plate to a signal frequency and to thecorresponding image frequency.

9. An image suppression system for super heterodyne radio receivershaving an antenna and a vacuum tube with a control electrode, includinga first resonant circuit, a portion of which is connected between saidcontrol electrode and ground; and a second resonant circuit in seriesbetween said antenna and said control electrode,

said resonant circuits having a common variable inductance devicecomprising an inductance coil, a ferro-magnetic core movable relativelythereto; and a conducting sleeve movable with said core relatively tosaid coil to vary the distributed capacity thereof, and being movable ina direction to increase the distributed capacity thereof with reductionof the inductance thereof, said sleeve being tapered in developed formin a manner to increase the rate of change of distributed capacity assaid sleeve moves toward said coil, said resonant circuits beingsimultaneously tuned by motion of said core and said plate to a signalfrequency and to the correspondin image frequency,

10. Tuning mechanism comprising, in combination, a coil, aferro-magnetic core movable into and out of said coil, and a conductingplate mechanically connected to said core and movable toward said coilas the core moves out said plate being split so as to minimize theinductance-changing effect thereof.

11. Tuning mechanism comprising, in combination, a coil, aierro-magnetic core movable into and out of said core, and a conductingplate mechanically connected to said 'core and movable toward said coilas said core moves out, said plate being tapered to provide a greaterrate of change of capacity as it moves toward said coil said plate beingsplit so as to minimize the inductance-changing elfect thereof.

12. Tuning mechanism comprising, in combination, a coil, aferro-magnetic core movable into and out of said coil, and a conductingsleeve mechanically connected to said core and movable toward said coilas said core moves out said sleeve being split so as to minimize theinductancechanging effect thereof.

13. Tuning mechanism comprising, in combination, a coil, aferro-magnetic core movable into and out of said coil, a conductingsleeve mechanically connected to said core for movement toward said coilas said core moves out, said sleeve being tapered in developed formwhereby the rate of change of capacity increases as said sleeve movestoward said core said sleeve being split so as to minimize theinductance-changing efiect thereof.

FREDERICK H. SCHEER.

